In an electrochemical gas sensor, the gas to be measured typically passes from the atmosphere into the sensor housing through a gas porous or gas permeable membrane to a working electrode (sometimes called a sensing electrode) where a chemical reaction occurs. A complementary chemical reaction occurs at a second electrode known as a counter electrode (or an auxiliary electrode). The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas (that is, the gas to be detected) at the working and counter electrodes. A comprehensive discussion of electrochemical gas sensors is provided in Cao, Z. and Stetter, J. R., "The Properties and Applications of Amperometric Gas Sensors," Electroanalysis, 4(3), 253 (1992), the disclosure of which is incorporated herein by reference.
To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte gas over the concentration range of interest.
In addition to a working electrode and a counter electrode, an electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode is used to maintain the working electrode at a known voltage or potential. The reference electrode should be physically and chemically stable in the electrolyte and carry the lowest possible current to maintain a constant potential.
Electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. The primary functions of the electrolyte are: (1) to efficiently carry the ionic current; (2) to solubilize the analyte gas; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode. The primary criteria for an electrolyte include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
In general, the electrodes of an electrochemical cell provide a surface at which an oxidation or a reduction reaction occurs to provide a mechanism whereby the ionic conduction of the electrolyte solution is coupled with the electron conduction of the electrode to provide a complete circuit for a current.
The measurable current arising from the cell reactions because of the analyte in the electrochemical cell is directly proportional to the rate of reaction. Preferably, therefore, a high reaction rate is maintained in the electrochemical cell. For this reason, the counter electrode and/or the working electrode of the electrochemical cell generally comprise an appropriate electrocatalyst on the surface thereof to enhance the reaction rate. If the reaction rate of either half cell reaction is impeded, resulting in a low exchange current density, the equilibrium current of the electrochemical cell may be easily perturbed during measurement. Such deviation can result in undesirable side reactions and/or nonlinear behavior over the range of analyte concentrations desired to be detected.
The type, rate, and efficiency of the chemical reactions within an electrochemical gas sensor are controlled, in significant part, by the material(s) used to make the working electrode and counter electrode. Indeed, extensive research efforts are expended to develop improved working electrodes, counter electrodes and electrochemical systems generally. See Cao, supra.
In the case of electrochemical sensors for the detection of hydrogen chloride (HCl), these efforts have met with somewhat limited success. In that regard, currently available electrochemical sensors for the detection of HCl suffer from a number of significant drawbacks, including: (1) poor response time; (2) short service time; (3) sensitivity to changes in temperature; (4) sensitivity to changes in humidity; and (5) susceptibility to interference from or cross-sensitivity to gases other than HCl (for example, hydrogen sulfide (H.sub.2 S)).
It is desirable, therefore, to develop new electrochemical sensors and electrodes for use in such electrochemical sensors for the detection of hydrogen chloride which mitigate or substantially eliminate one or more of the above drawbacks.